A System-on-Chip (SoC) integrates a microprocessor core with numerous interfacing circuitries on the same chip. Such interfacing circuitries, comprising receiver and transmitter pairs, allow the microprocessor core to communicate with other devices, such as memory, audio and video devices, and storage devices. Some of these devices may operate at a very high voltage. For example, Secure Digital (SD) card, which is a non-volatile memory commonly used in many portable devices, operates at 3.3v in default mode and outputs a signal which swings between 3.3v and ground.
SoC interfacing circuitries typically have voltage limiter circuits, which receive high-swing signals from the devices, reduce the swing to a pre-determined value, and send the modified signals to the receiver circuit. Voltage limiter circuits are needed because typically the receiver circuit, fabricated with a low voltage process technology which supports high speed operation but which has lower tolerance for voltage stress, cannot withstand the high-swing signals from the devices that operate at high voltage. Besides, to minimize leakage in the receiver circuit, the voltage limiter circuits' maximum output voltage typically matches or exceeds the supply voltage to the receiver circuit.
On the other hand, devices which the SoC interfaces with may also operate at different voltages for different modes of operation. For example, while SD card's signal swing is 3.3v at default mode, SD card may also have a high speed mode where the data rate doubles, and signal swing reduces from 3.3v to 1.8v. With a reduced input signal swing, the voltage limiter circuit output swing may reduce to a level below the supply voltage to the receiver circuit, which may result in substantial leakage in the receiver circuit. The reduced voltage limiter circuit output swing may also depend on temperature and process skew, which also adds uncertainty to the performance of the voltage limiter and to the amount of leakage in the receiver circuit.